The present invention relates to stereophonic FM broadcasting, and more particularly to a system for producing a 19 kHz pilot signal whose phase is automatically adjusted to correspond to the phase of the switching signal used for generating the composite stereo signal.
In stereo FM broadcasting, the composite stereo signal which is to be broadcast is usually generated by means for a switching modulator which alternately samples the left and right signals at a 38 kHz switching rate. A 19 kHz pilot signal is added to the composite signal for purposes of synchronizing the operation of the stereo demodulators which are provided in stereo FM receivers. For these demodulators to operate properly, the pilot signal must be in exact phase synchronism with the switching signal. In the prior art, this pilot signal was derived by first dividing down the 38 kHz switching signal to provide a 19 kHz square wave. A 19 kHz sine wave was then generated by filtering the 19 kHz square wave to remove all harmonic components above the fundamental. Unfortunately, when the 19 kHz pilot signal is generated in this manner, arbitrary phase shifts are introduced in the process so that phase synchronism between the 38 kHz switching signal and the pilot signal is hopelessly lost. Consequently, a phase adjustment was generally provided which was manually set so that the output signal of the filter was in exact synchronism with the 38 kHz switching signal. A system of this nature is described in the patent to Anderson et al. 3,789,323. This manual phase adjustment, however, was difficult to set in the first instance, and was prone to drift thereafter so that phase synchronism, even if achieved in the first instance, was later lost. This resulted in degradation of stereophonic separation of the left and right channels in subsequent demodulation of the composite signal.
The present invention provides a system wherein a pilot signal is produced which is in exact phase synchronism with the switching signal, and which does not drift from this desired phase relationship.
In accordance with the present invention, a circuit is provided for generating a switching signal and a pilot signal which is in exact phase synchronism therewith. The switching signal and a square wave pilot signal are derived in such a manner that the square wave pilot signal and the switching signal are in exact phase synchronism. The square wave pilot signal is then directed through a controllable phase adjuster circuit and into a filter. The filter serves to remove all harmonics above the fundamental so as to produce a sine wave pilot signal. The output of the filter is directed to a phase comparator which compares the phase of the sine wave pilot signal with the phase of the square wave pilot signal. The phase comparator provides a control signal which is used to adjust the phase correction provided by the phase adjuster so as to phase synchronize the sine wave and square wave pilot signals.
In accordance with another aspect of the present invention, a phase comparator is provided which is responsive to first and second input signals to provide an output signal which is responsive to any phase difference between the first and second input signals. A "lead" flip-flop which is included in the comparator is connected so as to provide an output pulse train wherein the width of the pulses provided thereby is dependent upon the amount by which one of the input signals leads the other input signal in phase. A "lag" flip-flop is also included and is connected so as to provide a second pulse train wherein the width of the pulses provided thereby is dependent upon the amount by which the one input lags the other input. An integration circuit is provided for integrating the difference between the two pulse trains to thereby provide the output signal.